SysRq

SysRq, also known as the Magic SysRq key, is a special key combination supported by the Linux kernel that allows users to perform low-level commands directly on the system, even when the system is unresponsive. It's a powerful debugging and recovery tool. To use SysRq, you typically press and hold the Alt and SysRq keys simultaneously, then press another key that corresponds to a specific command.

The available SysRq commands are numerous and cover a range of functions. Some common ones include rebooting the system (r), shutting down the system (s), dumping current registers (t), dumping all registers (p), and killing all processes except init (k). The kernel must be compiled with SysRq support enabled, which is usually the default. For security reasons, SysRq might be disabled or restricted by default on some systems, often through the sysctl parameter `kernel.sysrq`. Setting this parameter to a value between 0 (disabled) and 175 (all functions enabled) controls its behavior.

The primary use of SysRq is to regain control of a frozen or unresponsive system. For instance, if the graphical interface has crashed, SysRq can be used to safely reboot or shut down the machine, preventing data corruption that might occur with a hard reset. It can also be used for diagnostic purposes, such as triggering a crash dump or examining process states. Advanced users might employ it for kernel debugging or to force specific hardware actions.